In this section, we briefly revisit the preconditioner described in [1]}, for an isogeometric mass matrix associated with a single-patch domain, denoted \(\Omega \) , that is
\([\mathbf {M}]_{i,j}=\int _{\widehat{\Omega }} \widehat{B}_{i} \widehat{B}_{j} |\det (D F)|.\) 
